A group III nitride, for example, GaN (gallium nitride) based nitride semiconductor is expected as a material for a next-generation power semiconductor device. The GaN based semiconductor device has a wide band gap compared to Si (silicon), and can realize a high withstand voltage and a low loss compared to a Si semiconductor device.
A GaN based transistor is generally applied with a high electron mobility transistor (HEMT) structure in which a two-dimensional electronic gas (2DEG) is used as a carrier. A general HEMT is a normally-on transistor which comes to be a conduction state even when a voltage is not applied to a gate. There is a problem in that it is difficult to realize a normally-off transistor which does not come to be the conduction state as long as the voltage is not applied to the gate.
In power circuits handling large power such as several hundreds of voltage to a thousand voltage, a normally-off operation is required for the safety. Therefore, there is proposed a circuit configuration in which a normally-on transistor of a nitride semiconductor and a normally-off transistor of a silicon semiconductor are connected in cascode on the same substrate to realize a normally-off operation.
However, in such a circuit configuration, the heat generated by the normally-on transistor which consumes a large amount of power affects the operation of other elements. Therefore, there is a concern that the characteristics of the circuit may vary.